International Publication Number WO 2004/057921 A1 discloses a supply assembly for an LED (a light emitting diode) lighting module, including a control switch for supplying a constant current to the LED lighting module. A dual switching signal composed of low frequency bursts of high frequency pulses is applied to the control switch. By varying the low frequency component of the dual switching signal, the average current through the LED lighting module may be varied in order to vary the light intensity outputted by the LED lighting module.
In what is called a burst dimming system for dimming such that a load current intermittently flows through the LED lighting module, it is necessary to set the module to a comparatively low frequency (about 120 Hz) when the module is dimmed so that luminous flux from the module becomes low.
In the burst dimming system, there is a concern of visible flicker caused by interference with a specific frequency of video equipment such as a video camera when the LED lighting module is seen through the video equipment. Because a load current intermittently flows through the LED lighting module, thereby generating an idle period during which no load current flows through the module.
In an amplitude control system for varying a peak value of a load current through an LED lighting module, it is possible to reduce a possibility of generation of visible flicker when the LED lighting module is seen through the video equipment. Because, in the amplitude control system, a load current continuously flows through the LED lighting module.
However, if the amplitude control system is applied to the aforementioned LED lighting module having an LED array, there is a problem that elements (LEDs) of the LED array each have dispersion in brightness. Because the elements of the LED array have dispersion in a forward voltage, and may have 15% or more dispersion in some array or element types, and therefore a voltage applied to the LED lighting module may decrease when luminous flux from the module under dimming is low. The problem becomes more marked as the number of LEDs connected in series is increased.